Electrical devices embodying ferrielectric substances



Jan.23, 1968 c. F, PULVARI 3,365,400

` ELECTRICAL DEVICES EMEODYIN FERRIELECTRIC SUBSTANCES Filed March 14,1960 IN1/EN TOR.

" @www E Palm@ United States Patent O 3,365,400 ELECTRECAL DEVICESEMBODYING FERRIELEC'ERIC SUBSTANCES Charles Ferenee Pulvari, 2014rIaylor St. NE., Washington, D C. 20018 Filed Mar. 14, 1960, Ser. No.14,585 Claims. (Cl. 252-623) This invention relates to new substanceshaving ferroelectric properties and to electrical devices utilizing suchsubstances.

When ordinary ferroelectrics are compared with magnetic materials it isobserved that the most striking difference is the fact that unlikemagnetic materials ferroelectrics do not have a threshold switching-field, because dipole-dipole interaction perpendicular to the directionof polarization is weak as compared to the spin-spin interaction inmagnets in the same direction. ln single domains of ordinaryferroelectrics dipoles point in the same direction, as a result, theinteraction between dipoles side by side cannot be strong. Inantiferroelectric materials, opposite dipoles are side by side to eachother, therefore interaction between their opposite dipoles is strong.The interaction of antidipoles in antiferroelectrics is so strong thatno polarization reversal of practical usefulness is observed, and thenet spontaneous polarization in these materials appears to be zero.These facts led to the present invention.

According to this invention, it was visualized that if a material withan antidipole structure can be produced similar to an antiferroelectricmaterial, the dipoles of which however would be unequal, a new materialshould result with a strong dipole-dipole interaction comparable to thespin-spin interaction in magnetic materials and a threshold switchingfield should appear. This new type of Vmaterial was first discoveredwhen two antiferroelectric materials were in a solid solution. In thefollowing the name ferrielectrics, analogous to its ferrimagneticcounterpart, will be used to identify this new group of materials orsubstances. On a macroscopic scale below the Curie temperature theelectrical behavior of ferrielectrics is essentially similar to thatwhich is associated p with ferroelectrics. However, on a microscopicscale the sub-lattices do not contain equal and identical numbers ofdipole moments as do the antiferroelectrics, and a net spontaneouspolarization results. In general, materials exhibiting ferroelectricproperties composed of sub-lattices with interlaced unbalancedanti-dipole structure, representing an incompletely compensatedantiferroelectric material, will be referred to as ferrielectricsregardless whether they are composed of two different antiferroelectricsor they have said structure genuinely.

According to this invention two antiferroelectric materials are broughtin a solid solution which form then a ferrielectric material, with aninterlaced antidipole struc- .f

ture and incompletely compensated antiferroelectricity. The internalfields of such a structure combine so as to represent a thresholdswitching field. This special structure justifies the nameferrielectricity.

Because ferrielectrics differ from ferroelectrics in their microscopicstructure they have a number of heretofore not observed interestingelectrical characteristics.

The favorable switching properties of these new materials render themadaptable to a number of heretofore not feasible practical applications.Among these as examples are mentioned their use as a condenserdielectrics and as piezoelectric elements. For these purposes,ferrielectrics may be used in the form of single crystals or in the formof polycrystalline materials such as ceramics. Other applications arethe uses of such condensers as improved transpolarizers, informationstorage elements and matrices, logical circuits, etc. Anotherapplication is 3,365,400 iyatented dan. 23, 1968 rice the use of thesematerials as improved piezoelectric elements for sensing variousphysical properties such as pressure, temperature, volume changes, ingeneral, their use as transducers. Again, another application of thesenew materials is their combined use as a condenser dielectric andpiezoelectric element, for example, in an electromechanical lter-networkutilizing the unusual dielectric qualities of these new ferrielectrics.

Among the unusual electrical properties of ferrielectrics it was alreadymentioned that their switching characteristic exhibits a highernon-linearity and a more pronounced threshold switching field thancommon ferroelectrics. Their switching properties are similar tomagnetic materals and switching transients appear only beyond a certainfield strength. FIGURE l shows switching characteristics of twodifferent electrical condensers, one of which comprises commonferroelectrics as a dielectric, while the other one comprises the newferrielectrics as a dielectric. The applied switching field E is plottedon the abscissa, and the maximum values of the switching currentsmeasured on a load resistance are plotted on the ordinate. The dashedline 10 -represents the characteristic switching behavior of heretoforeknown ferroelectrics, and the solid lines 11 represent the switchingbehavior of the newly invented ferrielectric dielectric and show ahigher order nonlinearity than ordinary ferroelectrics. The switchingbehavior of ferrielectrics can not be represented by the simpleexponential relationship:

found for heretofore known or ordinary ferroelectrics (I. Appl. Phys.29, 1315-1321 (1958), instead an initial coercivity, i.e., a thresholdswitching eld E0 has to be considered below which switching practicallydoes not occur. This threshold ield was heretofore not observed inordinary ferroelectrics and as a consequence it limited their practicalusefulness.

Since a number-of these ferrielectrics are composed ofantiferroelectrics possessing a rather high Curie temperature (375 C.),a considerably improved domain stability was obtained in single crystalsas compared with ordinary ferroelectrics Domains once aligned do notchange into other type of domains as easily as they do in heretoforeknown ferroelectrics. Again, another advantage of ferrielectrics is thatsince it is a complex compound it lends itself for engineering varioustypes of ferrielectrics with properties desired for a given application.Coercivity as well as polarization of these materials can be variedwithin a wide region which is very desirable for a number of importantapplications.

The first ferrielectric material discovered was the heretofore notreported mixed crystal composed of two antiferroelectric materials,namely, sodium niobate and sodium vanadate:

The Curie temperature may vary depending on the composition, valuesobtained for various compositions are as follows:

This material exhibited well-saturated hysteresis loops from roomtemperature up to 225 C. This limit may be further improved when moreperfect crystals will be produced. Good quality single crystals could begrown within a wide 4range of compositions. The solid lines in FIG. 1show the characteristic switching behavior of this new material.Polarization, coercivity switching behavior can be varied within a widerange depending on the cornposition.

Another mixed crystal composed of two antiferroelectric materials,namely, sodium niobate and silver vanadate is also ferrielectric.

Another example of this novel ferrielectric group is composed of the twoantiferroelectric lead zirconate and lead hafnate:

4with a Curie temperature of about 210 C. Here again the hafnium atomtits very well into the zirconate structure similarly as vanadium doesin the niobate structure. Wellsaturated hysteresis loops are againobtained from room temperature up to the Curie temperature.

Other combinations of antiferroelectric materials resultiug in aferrielectric are:

A further modification of ferrielectric materials is obtained when smallamounts of impurities are introduced in the crystal in order to causechanges in the various properties or behavior, such as, for example, toinfluence the conductivity of the crystal or the crystal growth habit,etc. One method by which crystals of the ferrielectric substances ofthis present invention have been prepared is by reacting: 2.1 624 gm.(0.02 M) of sodium carbonate, 0.3638 gm. (0.002 M) of vanadiumpentoxide, and 4.7844

i gm. (0.018 M) of niobium pentoxide, as follows:

This mixture, with the addition of some acetone, was thoroughly groundin a tungsten carbide lined electric grinder and was then placed in asmall platinum crucible and loaded, at room temperature, into thefurnace. The temperature was then raised to l390 C. and a soaking timeof two and one half hours was allowed. Cooling was done on a 5 degreesper hour down to 1300o C. After this, the cooling rate was increased to30 degrees per hour down to 1200 C., after which a more rapid coolingended the cycle.

The crystals were cubic-like, light brown with clear-cut shiny faces andhad easy cleavage planes.

Thin plates which have been cleaved from larger cubes and have beenprepared for investigation show large homogeneous domain areas, whichcould be observed in cross-polarized light under a microscope. It wasfor the rst time that in mixed crystals large uniform domain structurewas obtainable. It is noted that this good domain structure was quiteabundant and even from a small batch a large number of good samplescould be produced.

. Crystals were also prepared by reacting at 1200o C. a mixture havingthe weight proportion of the desired composition. For illustrationpurposes, a sodium vana- Y date and 85% sodium niobate mixture is given.This calcined material was finely ground and packed in a platinumCrucible with sodium iiuoride ux in a ratio of i The furnace was heatedagain to ll00 C. and the mixture soaked for three hours, after which a 5degrees per hour cooling rate was used until 1000 C. The flux was thenpoured off and cubic like crystals were obtained.

FIGURE 2 is a perspective view of an electrical device which is usefulas a condenser or as a piezoelectric element and comprises theferrielectric materials of this invention.

FIGURE 3 is a perspective view of a similar electrical device as shownin FIG. 2 with two condensers.

FIGURES 4 and 5 exemplify other configurations of condensers useful aselectrical capacitors or as piezoelectric elements comprising theferrielectric materials of this invention.

FIGURE 6 represents an example of a multicondenser device embodying theferrielectric substances of this invention.

In the devices shown in FIGS. 2-6, the novel ferrielectric `body of thisinvention is shown in form of various shapes of slabs or circular discs(FIGS. 4 `and 5). The electrodes are adherent metal coatings l2 and 13formed on the opposite sides of the bodies 14. Lead wires 15 and 16 areelectrically connected to the electrodes for example by soldering. Thedevices shown in FIGS. 2-6 may serve, when properly utilized, as anelectric condenser or as a piezoelectric device.

When used as an electric condenser, the device makes use of the highdielectric constant,` and the novel heretofore not available dielectricproperties of these new ferrielectric materials.

In order to expose the dramatic differences between ordinaryferroelectrics and ferrielectrics, it is sufficient to refer to a few ofthe numerous novel electrical properties already mentioned. The unusualhigh nonlinearity of the switching characteristic with a thresholdswitching field, domain stability, the fact that polarization andcoercivity can -be varied within wide limits depending on thecomposition of these materials are all contributing to produce improvedcapacitors for various purposes.

When the devices shown in FIGS. 2-6 are used as a piezoelectric element,it may be operated While subjected to a constant direct-current biasingfield. j

When subjected to such a field, the bodies 14 exhibit piezoelectricproperties in that it changes in physical size in response to changes ofa potential applied across the body/ in a direction having a componentparallel to the direction of the biasing field, and in that, whensubjected to mechanical stress, it generates a potential, in thedirection of the biasing field, which varies with variations in ltheapplied stress. The effectiveness of the piezoelectric element increasesas the superimposed direct-current field 1s increased.

The direct-current biasing field for piezoelectric use may beestablished by maintaining a direct-current Voltage across theelectrodes 12 and 13 While the device is in use. A similar result can beachieved by subjecting the ferrielectric body to a high direct-currentpotential gradient for a substantial period of time prior to use. Uponremoval of this direct-current potential, a residual polarizationremains in the body which can be used as the source of the requisitedirect-current field without the use of an externally applieddirect-current potential.y Good permanent polarization can be induced inferrielectrics because of their threshold field, i.e. high coercivity.The residual polarization may be obtained more effectively if the bodyis heated to a temperature above the Curie temperature and is thenallowed to cool to room temperature under a high direct-currentpotential gradient.

The devices of FIGS. 2 to 6, when operated with an adequatedirect-current bias either externally applied or vresulting from remnantpolarization in the `bodies 14,

`.ay be used for any of the known piezoelectric purposes. Thus, it maybe used in the usual manner as a frequency control device or as anelectromechanical filter, the alternating-current voltage being appliedacross the leads and 16 and the external direct-current biasing voltage,if any, being applied across the same leads. A suitable externallyapplied direct-current biasing gradient may be between about 2,000 voltsper centimeter and 40,000 volts per centimeter.

The device may also be used in the usual manner as an electromechanicaltransducer where it is desired to convert variations of electricalcurrent or potential into corresponding mechanical variations, or viceversa, as in supersonic sound generators, microphones, telephonereceivers, phonograph pick-ups, piezoelectric relays and similardevices. In such devices, the usual mechanical means are supplied foreither transmitting mechanical energy to the body 14 as in microphonesand phonograph pick-ups, or utilizing the mechanical energy generated inthe body, as in supersonic generators, telephone receivers and relays.

As mentioned above, the body 14 of FIGS. 2 to 6 may be formed of theferrielectric substance in the orm of a single crystal or crystalsection or in the form of a coherent polycrystalline body, such as aceramic body prepared by sintering together finely divided particles ofthe ferrielectric crystals.

The devices of the present invention have been described as made upessentially of electrodes; spaced by a coherent body of one or moreferrielectric crystals. These devices may be manufactured according tothe techniques known in the art for the manufacture of analogous devicesembodying other ferroelectric crystal bodies. The best results areobtained when the electrodes consist of an adherent conductive coatingformed directly on the ferrielectric body, as by the application of aconventional silver paste, which is later red to produce an adherentdurable solid conductive coating, or by applying a sprayed or evaporatedmetal layer.

It may obviously be desirable to form the devices of the presentinvention with more than two electrodes in some instances. When theferrielectric body is in a polycrystalline substance, it may obviouslybe readily formed into various shapes other than those shown in thedrawings. Thus, it may be formed in the shape of a tube having aninternal metal coating and an external metal coating as electrodes or itmay be formed as an annular ring having suitably disposed electrodes.

The invention has been described above in terms of its specic embodimentand, since modiiication and equivalents will be apparent to thoseskilled in the art, the description is intended to be illustrative of,and not a limitation upon, the scope of the invention.

I claim:

1. A ferrielectric body having fer-roelectric properties, said bodyconsisting essentially of nely divided crystal particles including atleast two antiferroelectric substances, each of said substances havingthe formula A1303 wherein A is a member selected from the groupconsisting of sodium, silver, and lead, and B is a member selected fromthe group consisting of niobium, vanadium, zirconium, and hafnium.

2. A ferrielectric body having ferroelectric properties, said bodyconsisting essentially of a plurality of mixed crystals composed ofsodium niobate and sodium vanadate.

3. A ferrielectric body having ferroelectric properties, said bodyconsisting essentially of a piuralilty of mixed crystals composed ofsodium niobate `and silver vanadate.

4. A ferrielectric body having ferroelectric properties, said bodyconsisting essentially of a plurality of mixed crystals composed of leadzirconate and lead hafnate.

5. A ferrielectric body having erroelectric properties, said bodyconsisting essentially of a plurality of mixed crystals composed ofammonium dihydrogen phosphate and ammonium dihydrogen arsenate.

6. A ferrielectric body 'having ferroelectric properties, said bodyconsisting essentially of a plurality of mixed crystals composed ofammonium dihydrogen phosphate and deuteroammonium dideuterium phosphate.

7. A ferrielectric body having ferroelectric properties, said bodyconsisting essentially of a plurality of mixed crystals composed ofammoniumv dihydrogen arsenate and deuteroamrnonium dideuterium arsenate.

8. The method of making a ferrielectric body having erroele-ctricproperties which comprises the steps of mixing together sodiumcarbonate, vanadium pentoxide, and niobium pentoxide, heating themixture to a predetermined temperature, maintaining said temperature fora predetermined time, and then gradually cooling the heated materialuntil it becomes crystalline.

9. The method of making a ferrielectric body having ferroelectricproperties which comprises the steps of mixing together sodium Vanadateand sodium niobate, adding a metal uoride ux to said mixture, heatingsaid mixture and iiux to a predetermined temperature, maintaining saidtemperature for a predetermined period of time, cooling the heatedmaterial gradually to a predetermined temperature, and then pouring olfsaid ux when said cooled material becomes crystalline.

10. An electric device comprising at least two conducting electrodesspaced by a ferrielectric body composed ot a plurality of mixed crystalsconsisting essentially of at least two antiferroelectric substances,each of said substances having the formula ABO3 wherein A is a memberselected from the group consisting of sodium, silver, and lead, and B isa member selected from the group consisting of niobium, vanadium,zirconium, and hafnium, said ferrielectric body having ferroelectricproperties.

References Cited UNITED STATES PATENTS 2,911,370 11/1959 Kulcsar 252-6292,960,411 11/1960 Brajer et al.

2,976,246 3/ 1961 Egerton et al. 252-629 2,906,710 9/1959 Kulcsar 310-82,928,032 3/ 1960 Daniel 317-262.

OTHER REFERENCES Goldsmith et al., Ferroelectric Behavior of Thiourea,Journal of Chemical Physics, vol. 3l, November 1959, pp. 1175-1187; 1187most pertinent.

Megaw, Ferroelectricity in Crystals, Mehuen & Co., Ltd., 1957, pp. 110,111, and 121.

TOBIAS E. LEVOW, Primary Examiner.

SAMUEL yBlilRNSTEIN, JOHN L. BURNS,

MAURICE A. BRINDISI, Examiners.

J. D. KALLAM, I. S. RAPPAPORT,

R. D. EDMONDS, Assistant Examiners,

1. A FERRIELECTRIC BODY HAVING FERROELECTRIC PROPERTIES, SAID BODYCONSISTING ESSENTIALLY OF FINELY DIVIDED CRYSTAL PARTICLES INCLUDING ATLEAST TWO ANTIFERROELECTRIC SUBSTANCES, EACH OF SAID SUBSTANCES HAVINGTHE FORMULA ABO3 WHEREIN A IS A MEMBER SELECTED FROM THE GROUPCONSISTING OF SODIUM, SILVER, AND LEAD, AND B IS A MEMBER SELECTED FROMTHE GROUP CONSISTING OF NIOBIUM, VANADIUM, ZIRCONIUM, AND HAFNIUM.